1. Field of the Invention
The present invention relates to a process for the preparation of N-(hydroxyphenyl)maleimide copolymers and further relates to a novel copolymer belonging to the N-(hydroxyphenyl)maleimide copolymers.
2. Description of Prior Arts
N-(hydroxyphenyl)maleimide copolymers are soluble in organic solvents such as dimethylformamide, cyclohexanone, tetrahydrofuran and ethylcellosolve, and accordingly are of value as material for forming a thin coating layer. Moreover, since the N-(hydroxyphenyl)maleimide copolymers have phenolic hydroxyl groups and hence are soluble in an aqueous alkaline solution, these polymers are of value as base polymers of light-sensitive resins. Further, the N-(hydroxyphenyl)maleimide copolymers find various uses as heat-resistant resins.
Known processes for preparing N-(hydroxyphenyl)maleimide copolymers are described below.
Kobunshi Kagaku (Polymer Chemistry, in Japanese), Vol. 26, pp. 598(1968) describes a process for preparing N-(p-hydroxyphenyl)maleimide-styrene copolymer containing a relatively large amount of repeating unit derived from N-(p-hydroxyphenyl)maleimide which comprises copolymerizing N-(p-hydroxyphenyl)maleimide monomer (which has been prepared through transesterification) and styrene.
Preprint of the 7th Photopolymer Conference (held at Ellenville, N.Y. 1985) describes at page 35 a process for synthesis of N-(p-hydroxyphenyl)maleimide-styrene copolymer (1:1 molar ratio copolymer) comprises copolymerization of N-(p-acetoxyphenyl)maleimide monomer and styrene and subsequent transesterification of the resulting copolymer.
These known processes for the preparation of N-(p-hydroxyphenyl)maleimide-styrene copolymer require transesterification at a stage prior to or after the copolymerization in which an esterified hydroxyl group is converted into hydroxyl group. This is because an imide compound having hydroxyl group such as the above-stated N-(p-hydroxyphenyl)maleimide monomer is hardly prepared by conventional processes for the preparation of imide compounds.
As the conventional process for the preparation of imide compounds, there is well known a process comprising reaction of an amino compound and maleic anhydride in the presence of a basic catalyst such as triethylamine or sodium acetate and an acid anhydride-type dehydrating agent such as acetic anhydride, propionic anhydride or benzoic anhydride. This known process is not appropriately applied to the preparation of maleimide monomer from aminophenol and maleic anhydride, because hydroxyl group of aminophenol is predominently esterified and hence the desired imide compound having hydroxyl group is not produced in a high yield.
For the reason, the known process for the preparation of N-(p-hydroxyphenyl)maleimide-styrene copolymer involves transesterification of maleimide having an esterified hydroxyl group as described in Belgian Pat. No. 613,801, whereby returning the esterified hydroxyl group to hydroxyl group at either stage where it is in the form of a monomer or a polymer.
Thus, the known process for the preparation of N-(p-hydroxyphenyl)maleimide-styrene copolymer requires a stage of transesterification for returning the eserified hydroxyl group to hydroxyl group prior to or after the copolymerization reaction. The transesterification reaction needs a long time of period, and hence the known process is not favorably utilized in industry due to economical disadvantage. Another disadvantageous aspect of the known process resides in that the desired copolymer of maleimide monomer and styrene is not produced in a high yield because the double bond of maleic anhydride is attacked by the aminophenol in the stage of the production of maleimide monomer from the aminophenol and maleic anhydride.